This invention relates generally to digital output drivers for integrated circuits. More particularly, it relates to a circuit for changing the impedance of a calibrated output driver so that it may be used in multiple applications.
Calibrating the impedance of an output driver on an integrated circuit (IC) can have several advantages. It can reduce reflections on the output signal, reduce electromagnetic interference (EMI), reduce power dissipation, reduce signal skew, and provide termination impedances. In addition, some commonly used signaling schemes such as GTL+ (Gunning Transceiver Logic+) rely upon controlled impedance output drivers to achieve specified voltage swings.
Unfortunately, multiple desired controlled impedances may be necessary because the desired controlled impedance of an output driver may depend upon the location of the driver on a bus, or the type of bus being driven. For example, a GTL+ bus with a 50xcexa9 characteristic impedance can be terminated with 50xcexa9 to Vtt internal to the IC at each end of the bus. This can be done by using a controlled impedance pull-up at 50xcexa9. The 50xcexa9 pull-up is a first desired controlled impedance.
To achieve specified voltage swings, drivers that are located in the middle of the bus may be set to pull-down to ground with an impedance of 12.5xcexa9. This 12.5xcexa9 pull-down is a second desired controlled impedance.
Twenty-five ohms of pull-down impedance is desired at the end of the bus because, to save power, it is also desirable to turn off the 50xcexa9 termination internal to the IC""s at each end of the bus when those IC""s are driving. This is a third desired controlled impedance. Finally, a 50xcexa9 pull-down is a fourth controlled impedance that would be desired if the same driver were to be used with a source-terminated bus having a characteristic impedance of 50xcexa9.
Accordingly, there is a need in the art for a controlled impedance driver that can easily change it""s drive impedance for use in multiple applications. It is desirable that each drive impedance be able to use the same calibration information as the other drive impedances. This simplifies the design of the IC because a single set of calibration information can be distributed and used by different drivers to produce the appropriate drive impedance for it""s desired application. Finally, it is desirable that only a minimum number of control signals are necessary to switch between impedances.
In a preferred embodiment, the invention can change the drive impedance of a controlled impedance driver for multiple applications using only one or two control signals. The changes can be set at design time to allow a single driver design to be used on a source-terminated bus, as the termination on a terminated bus, or in the middle of a terminated bus. For each of these applications, the same driver design is used, and each instance of the driver design uses the same calibration information.
An embodiment of a driver according to the invention includes a multiplexer that shifts the calibration bits to the pull-down transistors. This shifting changes which transistors of a split drive transistor are turned on when the pull-down drive transistors are driving. By changing which transistors are turned on, the pull-down impedance of the driver is changed. This shifting is used with a disable function on the pull-up drive-transistor to allow the driver to be used as an end-of-line termination and driver, an open-drain driver, or as a source-terminated driver.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.